1. Field of the Invention
This invention relates to video signal processing circuits. More particularly, but not exclusively, this invention relates to video signal processing circuits which are suitable for use in special effects equipment in high definition video systems.
2. Description of the Prior Art
The standard television signal transmitted in the United Kingdom is a PAL signal of a 625-lines per frame, 50-fields per second system, and the PAL, NTSC and SECAM signals transmitted in other countries use similar or slightly lower line frequencies (for example 525 lines per frame), and similar or slightly higher field frequencies (for example 60 fields per second). While there is no immediate prospect of significant changes in the transmitted signals, there is an increasing requirement for higher definition video systems. Such systems can be used, for example, in film-making, in closed-circuit television systems, in satellite communication systems and in studio use generally. One such proposed high definition video system uses 1125 lines per frame and 60 fields per second. This proposed system also uses a 5:3 aspect ratio instead of the 4:3 aspect ratio now usual for television receivers.
The special effects which can be applied to a video signal are well known. Thus, for example, images on a cathode ray tube can be off-set (moved in any direction), scaled (expanded or compressed in size), rolled (rotated in two or three dimensions) and so on. Some of these special effects effectively involve treating the video signal as if it existed in three dimensions, the third dimension, that is the dimension normal to the plane of the television screen generally being referred to as the z-dimension. Where the z-dimension is involved, one particular problem is in eliminating hidden surfaces. Software algorithms for eliminating hidden surfaces using a z-buffer are known and are used when generating images such as in computer-aided design. See, for example, "Computer Display of Curved Surfaces" by Edwin Catmull of New York Institute of Technology. However, when using such algorithms in a computer, the necessary calculations take up to seconds to complete, making this implementation unsuitable for the real time operation necessary in a video system, particularly a high definition video system, where only nanoseconds are available for processing the information.